The present invention relates to a liquid crystal display (LCD) adopting the alignment layer, and more particularly, to an alignment layer having an excellent alignment property and excellent thermal stability, a method for forming the same and a LCD having the alignment layer.
Generally, a liquid crystal has both the fluidity characteristic of a liquid and the optical property of a crystal, and an optical anisotropic property thereof changes by an electrical field or heat. An LCD is a display device using such properties of the liquid crystal.
FIG. 1 is a sectional view showing the structure of a general LCD.
Referring to FIG. 1, transparent electrodes 13 and 13' are formed on upper and lower transparent substrates 12 and 12', respectively, and insulation layers 14 and 14' and alignment layers 15 and 15' are formed on the transparent electrodes 13 and 13', respectively.
Also, Spacers 16 are distributed between the upper transparent electrode 12 and the lower transparent electrode 12' to maintain a cell gap of a predetermined distance, and a liquid crystal layer 17 is formed in the cell gap.
Polarization plates 11 and 11' for polarizing incident light are formed at the outer surfaces of the substrates 12 and 12', respectively.
In the LCD having the above structure, performances such as light transmittance, response time, viewing angle and contrast are determined according to the alignment properties of the liquid crystals. Thus, it is very important to evenly control the alignment of the liquid crystals.
It is difficult to attain uniform alignment of the liquid crystals by simply interposing the liquid crystal layer between the upper and lower substrates. Accordingly, the alignment layers 15 and 15' are formed on the transparent electrodes 12 and 12' as shown in FIG. 1.
The alignment layer is mainly formed by forming a thin film of organic polymer such as polyimide and polyamide on the substrate, curing the thin film, and then rubbing the resultant using a rubbing cloth.
The above method for forming the alignment layer is easy and simple. However, the alignment layer may be contaminated by fine particles or fibers of the cloth used for the rubbing process. Also, insufficient alignment may be resulted according to the material of the alignment layer, and a thin film transistor (TFT) may be damaged by static electricity generated during the rubbing process.
In order to solve the above problems, an optical alignment method which does not cause dust or static electricity has been developed. According to such a non-destructive alignment method, polarized light is irradiated onto the optical alignment layer to cause anisotropic photopolymerization. As a result, the optical alignment layer has alignment characteristic, thereby uniformly aligning the liquid crystals. The polymer for the optical alignment layer includes polyvinylcinnamate (PVCN) and polyvinylmethoxyxinnamate (PVMC). However, such polymers have poor thermal stability, although they have an excellent optical alignment property. In other words, the thermal stability of an alignment layer depends upon that of the polymer, which depends upon a glass transition point and cross linking density. Since the polycinnamate generally has a glass transition point of 100.degree. C. or below and is difficult to subject to a photo-reaction of 50% or more in a solid state, increasing the cross linking density is limited, which weakens the thermal stability of the alignment layer.